Material handling equipment is available in a wide variety of types including, for example, lift trucks, excavators, backhoes and cranes. The latter type includes overhead travelling cranes (OTC) which ride on spaced railroad-like rails mounted above a surface, e.g., the floor of a room. Another type of crane which has one or two stilt-like legs is referred as a gantry crane (having two supporting legs and resembling an inverted "U" in shape) or a half-gantry which is shaped like an inverted "L" and has one leg supported at ground level and the horizontal portion supported by an elevated rail. While the invention is described in connection with an OTC, persons of ordinary skill will understand how to apply its teachings to other types of cranes and even stationary hoist systems.
A typical OTC has a bridge which spans the rails and which has a trolley atop it. The trolley travels on spaced rails mounted on the bridge structure. Mounted on the trolley is a hoist system having a rotating drum powered through gearing by an electric motor.
Such system also has what is known as a "bottom block" or "load block" suspended from cable and equipped with a hook or other device for lifting a load. A load block has one or more pulley-like, rotatable sheaves over which cable runs. While not common, dual-hoist systems involving two simultaneously-operating hoist drums "cabled" to a single load block are not unknown. Insofar as is known, such hoist systems have a single control for operating two hoist motors in parallel, one attached to each hoist drum. In other words, it is not possible to operate a hoist drum independent of the other hoist drum.
The crane is able to travel the length of the bridge rails (along the length of a room, for example) and the bridge-mounted trolley can travel the width of such room. Therefore, by properly manipulating crane and trolley position, the operator can "pick" a load from about any location in the room and move it to any other location.
Many (indeed, perhaps most) applications for overhead travelling cranes are not so critical that they require redundant hoist systems. If a hoist system fails, operations are merely suspended for the time necessary to effect repairs. On the other hand, there are certain applications, usually involving some sort of hazard, where the potential risk justifies provision of a redundant system.
An example of an application of the latter type is a nuclear power station. There, fuel "charges" (bundled rods of radioactive nuclear fuel ready for placement in a reactor) need to be handled expeditiously as does the spent but still radioactive fuel residue removed from a reactor. It is sometimes not possible or desirable to leave a load of radioactive material at a location where humans might be exposed thereto while hoist repairs are undertaken.
As with other types of hoist systems, the matter of a primary and a redundant hoist system cabled to a single load block involves (or should involve) considerations of what is known as "fleet angle." The fleet angle is the included angle between a sheave-engaging cable and the plane of sheave rotation. Explained in different terms, when cable is aligned with the groove in a sheave across which the cable travels, the fleet angle is substantially zero. On the other hand, when cable is "cocked" and enters or leaves the sheave at an angle, this is understood to involve a fleet angle which is other than zero.
An excessive fleet angle can cause any one or more of several problems. One is that cable rubs on one of the pulley rims. Undue (and, in view of the invention, unnecessary) cable and rim wear result. Another problem is that in a more severe case involving very excessive fleet angle, cable may jump out of the pulley groove and "roll over" the pulley rim. Such an eventuality usually immediately disables the hoist system.
To explain the source of another type of difficulty, it is assumed that a hoist system has fully lowered a load and thereupon becomes disabled. It is also assumed that the system uses two "parts" of cable, a common arrangement providing a 2:1 lifting advantage, and is further assumed that the cable is wound on the drum in a way that the points of cable tangency become spaced further apart as the load lowers.
In this situation, it is apparent that if the load block is raised toward the main hoist drum using means other than by rotating such drum, the fleet angles increase. Such increases can lead to problems as described above.
The invention addresses these problems and shortcomings in unique and imaginative ways.